Electromagnetic induction apparatus

ABSTRACT

Electromagnetic induction apparatus for high-voltage, high-power operation, using a compact interrupted magnetic circuit whose materials and geometry are so designed as to confine magnetic flux to the compact geometry associated with the magnetic circuit, to confine operating electric currents to windings which cooperate in forming the magnetic circuit, and to distribute both normal and transient voltages quickly and evenly along both the winding and the magnetic circuit so that the electric field strength of the system is maintained high. The apparatus may be applied to either the transformer or the reactor function, but in either event the purpose and advantages of the invention generally relate to so-called large power operation (above ten megavolt amperes) at voltages in the EHV and UHV levels. EACH ODDDDZ0Dsre etdDDD)s nDDDE

United States Patent Trump et al.

ELECTROMAGNETIC INDUCTION APPARATUS Inventors: John George Trump,Winchester; Brian Skillicorn, Topsfield; Bryon Lee Johnson, Westboro,all of Mass.

High Voltage Power Corporation, Wesboro, Mass.

Filed: July 12, 1971 Appl. No.: 161,833

Related US. Application Data Continuation-in-part of Ser. No. 840,090,June 2, 1969, Pat. No. 3,593,243, which is a continuation-in-part ofSer. No. 567,641, July 25, 1966, abandoned.

Assignee:

References Cited UNITED STATES-PATENTS 12/1948 Hodnetle ..336/2l0 X10/1959 Lambenton ..336/212 10/1964 Miller ..336/60 X 3/1965 Deuron..336/70 X 51 Aug. 15, 1972 Primary Examiner-Thomas J. Kozma Attorney-Robert B. Russell et a1.

[ 5 7 ABSTRACT Electromagnetic induction apparatus for high-voltage,high-power operation, using a compact interrupted magnetic circuit whosematerials and geometry are so designed as to confine magnetic flux tothe compact geometry associated with the magnetic circuit, to confineoperating electric currents to windings which cooperate in forming themagnetic circuit, and to distribute both normal and transient voltagesquickly and evenly along both the winding and the magnetic circuit sothat the electric field strength of the system is maintained high. Theapparatus may be applied to either the transformer or the reactorfunction, but in either event the purpose and advantages of theinvention generally relate to so-called large power operation (above tenmegavolt amperes) at voltages in the EHV and UI-IV levels.

12 Claims, 31 Drawing Figures PATENTEDws 15 m2 SHEET 3 OF 8 llllll HFIG; 68

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PATENTEDAHG 15 972 saw u or 8 PRIOR ART PATENTEDausi 5:912 3.684.991

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- /9/ 30/ F/G. 23b ,86 ,89 m ,90 w 3 1 ELECTROMAGNETIC INDUCTIONAPPARATUS CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of our application Ser. No. 840,090, now US. Pat.No. 3,593,243, filed on June 2, 1969 which in turn was acontinuation-in-part of our then-pending application Ser. No. 567,641filed on July 25, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to deviceshaving time-varying magnetic flux and, more particularly, totransformers and reactors which use an insulated core.

As electrical devices have developed, different techniques have evolveddepending upon the magnitude of the various parameters involved. Forexample, high voltage d.c. equipment entails strong electric fields andhigh power a-c equipment has come to entail strong magnetic fields.Specialized techniques have been worked out for handling these strongelectric fields in high voltage d.c. equipment: for example, megavoltaccelerator apparatus employ hollow, rounded high voltage terminals andequipotential planes for controlling the electric field and shaping ituniformly. On the other hand, different specialized techniques have beenworked out for handling the strong magnetic fields of high power a.c.equipment: for example, ferromagnetic material is used to form magneticcircuits in which elaborate steps are taken to minimize reluctance andeddy currents. However, the need for these specialized techniques existsonly for certain ranges of certain parameters. For example, conventionalhousehold appliances do not require special techniques for shapingelectric fields, and high-voltage electrostatic accelerators do notrequire ferromagnetic material.

Conventional electric power techniques have evolved in a similar manner.Initial efforts involved do. and the requirements of average householdequipment, such as lighting, led to voltages of the order of volts. Withthe development of a.c. and transmission of electric power over greaterdistances at higher voltages to reduce losses in transmission, apparatuscapable of handling voltages of the order of 10 volts were developed,and related insulating and magnetic circuit techniques were devised.Such techniques'were limited to their own range, however, and the morerecent interest in even higher voltages for power transmission hastriggered a need for fundamentally new approaches and has generated newgeneric names: EHV (which stands for extra high voltage and includes therange 345-765 kilovolts in overhead systems and 230 kilovolts and up inunderground systems) and UHV (which stands for ultra-high voltage andincludes the voltages above 765 kilovolts now contemplated for overheadtransmission). At the extra-high voltage range, problems not beforefaced must be solved: The Ferranti effect becomes a problem intransmission lines, the strong magnetic fields required by transformersmust overlap the strong electric fields which must be controlled atthese high voltages, and other new problems arise.

The basic approach to the design of apparatus for coping with strongelectric and magnetic fields is disclosed in the insulating-core patentsof Van de Graaff.

The present invention is an improvement and refinement of that basicapproach in which insulating-core principles are applied toelectromagnetic induction apparatus for use with EHV and UHVtransmission lines for the absorption and storage of large quantities ofelectric charge associated with the capacitance of long transmissionlines, or in the transfer of power from one voltage level to another insuch large megavoltampere amounts.

Today, the pressing requirement for more and cheaper electrical powerfaces growing technical and esthetic problems including the nationaldesire to maintain the attractiveness of populated areas. To meet thiscontinuing need for more electric power without adding more generatingand transmission systems within cities and surburban areas, electricutilities are now building power plants in remote regions close to thesource of either large amounts of hydropower or large coal deposits.This power can be transmitted to load centers most economically byoverhead transmission lines. However, because of increased populationdensi ty and pressure to preserve the esthetic and economic valuesof thecountryside, transmission rights-of-way are increasingly difficult toobtain. The utilities are thus compelled to increase several-fold thepower transmitting capacity of their existing right of ways, and to planon still further increases in power in the future. For these and otherreasons, the electric power industry is rapidly converting to extra highvoltages (EHV) for the transmission of electric power having line-tolinevoltages in excess of 345 KV. Already SOOKV systems are in service, andmore recently 765 KV systems have been energized. Such high voltagespermit the transfer of larger blocks of power over extensive geographicareas. EHV interconnections are also used to even out demands over largeregions and to improve the reliability of the total system. Indeed, thetrend toward higher voltages is fundamental to meeting the predictablepower needs of the next decade.

Although there are important technologic and economic reasons for usingEHV, serious difficulties have been encountered in designing reliableterminal and line equipment for use at these high voltage levels. Thesimple extension of prior art concepts to EHV equipment is difficult andnew concepts in power handling equipment at EHV are clearly required.Especially needed are novel transformers and reactors capable ofreliable insulating performance at these extra high voltages andcharacterized by more efficient utilization of their materials andvolume.

In its simplest form, a transformer consists of two conducting coilshaving high mutual inductance. The primary winding is that coil whichreceives electric power and the secondary winding is that coil whichdelivers the power induced therein by currents flowing through theprimary winding. In normal practice these coils are wound on a core ofmagnetic material. In EHV transformers, the necessity of correspondinglyincreasing the insulation between the high voltage windings and thegrounded core adversely affects the operating characteristics, the cost,and the insulation reliability of the apparatus.

Additionally, it is essential that high power transformers built for EHVduty be designed to avoid or withstand the greatly increased forcesassociated with short circuits, voltage impulses, switching surges andthe like.

Attempts were made to solve these problems with prior art transformerdesigns by increases in the insulation dimensions.

Conventional prior art transformer design utilized a magnetic circuitformed by an iron core which was designed for minimum reluctance inorder that the magnetic circuit might indeed function as a circuit inwhich magnetic flux is confined as much as possible. Hence gaps ofnon-ferromagnetic material were avoided, and the magnetic circuit was ata common potential, generally ground.

Exceptions to this design principle are found in applications whichinvolve the transmission of only signal and instrumentation amounts ofpower. For example, high voltage measurement transformers need only tocreate in the secondary an accurate signal. Indeed absorption of powerfrom the circuit being measured is to be minimized. One specific designof measurement transformer utilized essentially an air-core choke-coil,and then obtained a signal by means of a small secondary coil linked tothe primary (choke) coil by a short length of iron core. The originalidea has been attributed to Biermanns, and various embodiments werebuilt by AEG. Some of the later embodiments were built by AEG. Some ofthe later embodiments inserted additional iron core members in theprimary (choke) coil, and publications concerning this show how this maybe done without impairing the accuracy of the device. However, none ofthe embodiments were designed for the transmission of high power (onepublication indicates upper limits of 40VA in one design and 100 VA inanother design) and none of the embodiments were designed for EHV (Thehighest voltage indicated is 220 KV). Representative publications.concerning this family of equipments are as follows: ElektrotechnischeZeitschrift 1931 Heft l2 (19 March 1931) pp. 378-379 ElektrotechnischeZeitschrift 58 .lahrg. Heft 8 (25 February 1937) pp. 203-205 StromundSpannungswandler by Walter (2nd ed. 1944)PP-98, 106-109 German Pat. No.732,281 (1943) Swiss Pat. No. 199,897 (1937) German Pat. No. 592,8761934) The first serious proposal for using insulating magnetic cores inpower devices was made by Van de Graaff and the present inventionrelates to improvements in the application to various a-c power devicesof the basic insulating-core principles disclosed by Van de Graaff in,for example, the following US. Pat. Nos.:

Suffice it to say that conventional extensions of prior art apparatushave encountered serious difficulties in high voltage power applicationsparticularly in the control of the electrical field distribution and thetendency toward unmanagable size and losses.

These difficulties are managed in a superior way by the presentinvention which provides a novel EHV transformer of high electrical andspatial (volumetric) efficiency. Moreover, the concept of this inventioncan easily be extended to handle even the higher Ul-IV levels that arepresently contemplated.

Large a-c magnetic core transformers, as known to the prior art, arehighly efiicient and practical power transforming devices whoseavailability has made possible the modern a-c power system. However,when a conventional design is operated at the extra high voltagecontemplated by this invention, the voltage insulation problem which canbe readily managed at low voltages becomes difficult and capable ofculminating in catastrophic breakdown.

Similarly, a reactor for electric power systems is primarily ahigh-voltage high-power inductance coil used'primarily to constitute alagging power factor load. For the most part such devices comprise acoil and a magnetic circuit so related as to exhibit high reactance withlow resistance. Reactors are usually used as shunt reactors on longlines to compensate for line charging current. With the advent of EHV,shunt reactors carry an increased importance. For example, in EHVsystems, leading currents due to line capacitance can cause excessivevoltages at the end of a long, lightly-loaded line. Unless prevented,these excessive voltages known as the Ferranti affect, can createinstabilities and subsequent failure in the terminal apparatus. Shuntreactors connected as required on the line end would have the desirableeffect of preventing these instabilities and failures.

The reactors known to the prior art generally were either of theso-called shell design or the gapped-core design. The shell designreactor consists of an air-core coil having a magnetic shell surroundingit, while the gapped-core design comprised a modification of this whichincluded an iron core within the coil which was intercepted by segmentsof stiff non-magnetic material.

In the shell design, the coils of large diameter and radial build-up aresubjected to a high leakage flux with resultant severe eddy currentloss. In the gapped-core design, the lower reluctance of the magneticcircuit generally results in lower winding losses but the voltageinsulation between winding and grounded core is rendered far moredifficult. Saturation of core iron must be avoided both to reduce lossesand to insure constant inductance over the entire operating voltagerange.

Thus, until the present invention was conceived, the design of EHVreactors was progressively more difficult and uncertain as to insulatingstrength, reliability, freedom from corona and radio noise. No clearlyadequate solution for these problems was discemable, especially whenoperating at their high voltage limits.

SUMMARY OF THE INVENTION Application of the principles of the presentinvention provides not only excellent distribution and control of thenormal a-c operating potentials but also excellent impulse voltagedistribution. The invention leads to relatively compact windings withshorter total conductor length and with correspondingly lower losses,and to substantial reductions in the required magnetic materials withstill further loss reduction.

The principles of this invention also contribute to the avoidance ofsaturation of elements of the magnetic I present invention whilesimultaneously reducing the overall size of the required unit and itscost. These benefits are realized in both transformer and reactorsconstructed under the present invention by utilizing the insulating coreconcept in which the active portions of the magnetic circuit are formedof electrically isolated segments, each electrically connected to itssurrounding coil, to provide systematic and uniform progression ofimposed voltage on both the active portions of the magnetic core and itsassociated electric circuitry.

DESCRIPTION OF THE DRAWINGS The invention will be best understood andappreciated from a perusal of the following description taken inconjunction with the figures wherein:

FIG. 1 shows in diagrammatic section view a conventional transformer.

FIG. 2 shows a diagrammatical section view of a transformer built inaccordance with the principles of the present invention.

FIG. 3 shows in detail one segment of the insulated core.

FIG. 4 shows in detail a lamination of the core of FIG. 3.

FIG. 5 shows detail of the arrangement of the transformer coils andcores together with the insulating disks.

FIG. 6A shows in section an insulating disc suitable for use in theinvention.

FIG. 6B shows in section an insulating disc suitable for use in theinvention.

FIG. 7 shows the invention used as a three phase transformer.

FIG. 8 shows in section, one type of prior art reactor.

FIG. 9 shows in section, a different type of prior art reactor.

FIG. 10 shows a cutaway view of a reactor built in accordance with theprinciples of the present invention.

FIG. 11 shows a partially broken away view of the operational element ofFIG. 10.

FIG. 12 is a view of the device of FIG. 10 taken along the lines 12-12.

FIG. 13 is a detail view of the operation element of FIG. 12 taken alongthe lines l3l3.

FIG. 14 shows further additional detail of the reactor of FIG. 10.

FIG. 15 shows an additional view of the operational element of FIG. 10.

FIG. 16 shows in schematic form wiring connection suitable for use inthe invention.

FIG. 17 shows the detail interconnection of the pattern shown in FIG.16.

FIG. 18 shows in schematic form, a different wiring connection suitablefor use in the invention.

FIG. 19 shows the detail interconnection of the wiring pattern on FIG.18.

FIG. 20 shows a possible modification of the equipotential hoops used inthe invention.

FIG. 21 illustrates a possible improvement that can be used with thepresent invention.

FIG. 22 shows a cutaway view of another reactor built in accordance withthe principles of the present invention, in which a thin, planarhigh-voltage terminal is employed.

FIG. 23a is a detailed view of several core elements of the reactor ofFIG. 22 in the vicinity of the thin, planar high-Voltage terminalthereof, including the various coils associated with these coreelements.

FIG. 23b shows graphically the potential decrease through the coilssurrounding each core element.

FIG. 24 is a plan view, partly broken away, of the high voltage terminalof FIGS. 22 and 23a.

FIGS. 25a, 25b, 25c and 25d are details of the high voltage terminal ofFIG. 24 showing certain steps in the formation thereof.

FIG. 26 is a view, similar to that of FIG. 3, showing a core element ofa reactor so constructed as to render it an equipotential member whilesuppressing eddy currents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning first to FIG. 1 whichshows in a section a conventional transformer, it can been seen thatsuch a transformer basically comprises a magnetic circuit 20 and a pairof current carrying coils 22 and 23 contained in a housing 24. In suchconventional transformers the magnetic circuit 20 usually consists of aclosed laminated core of hollow rectangular shape in which transversenon-magnetic gaps are carefully avoided.

Such transformers operate as follows. A time-varying magnetizing currentin the primary coil 22, connected to a source of ac power, produces asynchronously changing magnetic flux in the magnetic circuit which inturn induces an electromagnetic force in the secondary coil 23. Loadcurrent and power delivered by coil 23 is matched by the input toprimary coil 22 of a corresponding input current and power plusassociated losses. In order to minimize R losses the coils should bewound close to the magnetic circuit. However, the electricalconductivity and great mass of the core requires that the magneticcircuit be at ground potential; thus adequate voltage insulation isrequired between the core and the coils. As the operating voltages andtransient over-voltages of this type of apparatus are increased, therequired insulation must also be reliably increased. The coils arespaced progressive ly further from the core with an attendent increasein winding impedance and losses. Insulation barriers are introduced tocontrol the migration of space charge and electrified particulates. Theinsulation distances must be increased more rapidly than the ratedvoltage. These factors cause the physical size of both coils and core toincrease thus adding to the size, weight and losses of the unit.

The present invention restores the insulation harmony'between core andcoil by maintaining them as close to the same potential at all timesirrespective of the voltage rating of the apparatus. This is done byseparating the active, or winding-bearing portion of the magneticcircuit into core elements, mounting them in a stack or column with eachcore element electrically insulated from its neighbor by an adequate butrelatively thin layer of high quality dielectric. Each of theseinsulated magnetic elements has in close proximity around it aproportional share of the total winding, with the mid-pointor some otherpoint in this local winding being electrically connected to itsassociated core section and firmly establishing its potential at alltimes. In this way the stack of insulated cores follows closely thepotential distribution of the associated total winding and theelectrical incompatibility of winding and core which characterizesconventional transformer and reactor designs is almost totally avoided.

One embodiment of the present invention will now be described inconjunction with FIG. 2 which shows a step-down, single-phase,auto-transformer, constituting one phase of a three-phase Wye-connectedsystem, capable of handling extra high voltages and built in accordancewith the principles of the present invention. In this figure themagnetic circuit 30 comprises a pair of magnetic returns 31 and 32 whichcouple a pair of segmented legs 33 and 34 formed of a stack of magneticcore sections 35 and 38 electrically isolated from one another byinsulating disks 36.

Surrounding each leg is a pair of current carrying windings. Theauto-transformer shown consists of four series windings 41, 42, 43 and44 and four common windings 45, 46, 47 and 48. Series windings 41 and 42and common windings 45 and 46 are mounted on segmented leg 34 while theremaining windings are mounted on the other segmented leg 33. Eachwinding is composed of a plurality of current carrying coils 37 whichsurround a core section 35 and are electrically connected thereto.

A central core 38, without a coil, may be provided in each segmented legto separate the series windings and to serve as a means of introducingthe high voltage to the series windings. However, in a preferredembodiment of the invention, to be described in detail hereinafter, thehigh voltage is introduced via a thin, planar high voltage terminal.High voltage is supplied to the four parallel series windings and tocores 38 by high voltage lead 39 which is connected to a suitable acpower source (not shown).

The output voltage tap 40 is connected to the junction of the serieswindings and the common windings. The other lead from each commonwinding is, in turn, connected to the closest magnetic return 31 or 32,and to ground.

Turning now to FIGS. 3 to the details of one form of construction willbe described. In this embodiment each core section is made up from amultiplicity of rectangular silicon-steel laminations 50 formed with aplurality of holes 51 whereby the entire series of laminations making upthe core section may be united together. To avoid magnetic saturation atthe core section edges and to improve the electric field distributionthe narrow ends 52 and 53 of each lamination may be formed so that theinsulating gap increases toward the edge. After the laminations 50 areassembled, the corners of each core segment may be chamfered and theedges 55 and 56 ground to assume a profile similar to ends 52 and 53.Alternatively the core sections 36 could be formed from a single stripof material spirally wound. The actual dimensions of each core sectionare dependent on the power to be handled and can readily be determinedby one skilled in the art. A preferred form of core segment is describedin detail hereinafter.

Each coil, as shown in FIG. 5, is wound close to its respective coresegment.

The coils can be formed in two parts 57 and 58 from insulated conductingstrips spirally wound. In this embodiment coil half 57 is wound in onedirection while coil half 58 is wound in the other direction. The twohalves are then electrically connected to each other and to the adjacentcore section by an appropriate conductor 59. Adjacent coils are joinedby a suitable connector 60 to form the total winding. Thus the totalwinding is electrically connected to each of the core elements atrespective neighboring portions thereof so that the electric potentialof each core element is main tained close to the potential of thatportion of the winding nearest to the core element in question.

An insulating spacer 36 is provided between adjacent coil-core pairs soas to electrically isolate and insulate each coil-core pair from thecoil-core pair adjacent to it. The thickness of each of said insulatingspacer 36 should be sufficient to support a voltage of at least 2V/n,where V is the operating voltage of the high voltage lead 39 and n isthe total number of insulating spacers in each magnetic column or leg33, 34. Between each coil half may be interposed a second insulatingspacer 61 which may have afiixed thereto a support 62 for maintaining anequipotential ring 63 around the coils. Both the spacer and the support62 can be made of any suitable insulating material. Examples of suitablematerials are impregnated paperboard, plastic or inorganic sheets. Toprevent irregularities in the coils from creating high electricalstresses across the spacer 61 and mechanical damage, a semi-flexiblecoil spacer 65 can profitably be fitted around each coil pair 57 and 58.

As indicated previously most of the difficulties encounteredby the priortransformer art in attempting to produce reliable EHV transformerscentered around the necessity of separating and insulating the highvoltage winding from the grounded iron core through which the magneticworking flux flows.

The segmenting and electrical isolation of each core section fromadjoining sections solves the voltage insulation problem because eachcore section and its surrounding coil are at the same potential and noconflict in insulation strength exists between them. Thus the need oflarge spacings and heavy insulation between the coils and the cores isavoided. However, introduction of insulation into the gap tends toincrease the reluctance of the magnetic circuit which tends to increasethe requirement for high magnetizing, current and power andsimultaneously permits increased leakage of the magnetic flux. Increasedleakage flux results in increased reactance drop under load.

The reactance drop can be reduced by providing on each leg two completewindings in parallel so that for a given total power output per leg theload current per winding is halved. With this arrangement, as shown inFIG. 2 the highest voltage level is reached at the midpoint of each legand only a very small portion of the entire magnetic circuit is at thishigh potential. Although this arrangement approximately doubles theactive height of each transformer leg, our studies show that the overallcore utilization is actually improved because a larger proportion of thecore is actively used for winding purposes. Moreover, each magneticreturn is now at ground potential and can be designed and supported moreeconomically and effectively.

The insulated core and winding construction defined by this inventionhas further important advantages. It is important to note that eachinsulating spacer together with its adjacent core and coil constitutes acapacitor. Thus the active leg forms a series chain of capacitorsextending from the high voltage terminal by two parallel routes toground. By this series capacitance system better division of surgepotential is provided across the core and winding columns. This improvedsurge division occurs because the insulated, segmented coil-corearrangement provides for each current path between high voltage andground a series chain of high capacitances of nearly equal value. It ispractical for the first time to design this series capacitance system sothat transient over-voltages will be distributed with nearly exactuniformity along the entire stack. It is therefore expected that highBIL (Basis Insulation Level) and increased insulation integrity can beachieved under the use of the invention.

By controlling to greater uniformity the normal and transient electricalstresses, the apparatus can be designed for subcorona operation with theelimination of radio noise. It is to be noted that preferably each coil,core and insulator can be mechanically and physically identical in shapeand function to every other coil, core and insulator. The repeatedfabrication of identical subunits is expected to further reduce costsand improve quality.

The insulator 36 can be molded in the form of a disc with anti-flashoverconfigurations at its outer periphery. Two such designs are shown insection in FIGS. 6A and 6B. The disc of FIG. 6A comprises a planar discof insulating material 70 which is molded with a flared periphery 71 andcoated on both sides with a thin conductive layer of intermediateresistivity material 72. Typically this coating 72 will have aresistance of between 5,000 to 50,000 ohms per square, thus preventingexcessive eddy currents in this coating. A beading 73 is then moldedover the flare 71 and the extremities of the coating 72.

This smooth conductive coating 72 in intimate contact with the soliddielectric establishes the electric field boundary and thus preventsirregularities in the Iaminations of the cores from creating points ofhigh electrical stress. The conductive coating is designed to distributethe electrostatic potential uniformly across the entire surface of theinsulator 36 with controlled reduction of electric stress at the edges.

Referring to FIG. 6B, it is also possible to utilize an insulating sheet311 which has been rendered flat and parallel and which is compressedbetween the magnetic core elements 312 and between the coils. In thiscase it has been found desirable to include a sheet 313 of thin plasticand/or paper on each side of the insulating sheet 311 to permitmechanical accomodation of the two abutting surfaces and to reduce theeffects of unevenesses. No resistive coating is used in thisconfiguration which is shown in FIG. 6B.

In certain circumstances it would be desirable that the insulator 36 bemade in two parts. One part would be a central disc having aconfiguration as shown in FIG. 6A or 6B. The core segments would abuttthis part. The other portion would comprise a ring concentric with thisdisc which would isolate the coils from one another. This ring wouldalso have a sectional configuration such as shown in FIG. 6A or 6B. Thissplitting of the insulator .36 would have the beneficial result ofpermitting the cores and the coils to move in dependently of one anotherthus providing a more flexible design.

It should now become obvious to one skilled in the art that thisarrangement will also adapt itself for use with three-phase circuits. Insuch three-phase applications the structure might well appear as shownin FIG. 7. In this application three segmented core legs 80, 81 and 82are provided. Each leg carries the necessary number of coils and ismagnetically interconnected to the other two legs by magnetic returns83.

The present invention can also be advantageously used as a reactor.

Shunt reactor needs are determined by the length of the transmissionline, its loading, and the general problem of var control. They are alsoquite effective in helping to limit transient overvoltages. On many EHVsystems, generator reserve requirements or pool reserve requirementswill be such as to cause the lines to be on standby service much of thetime. In such cases, shunt reactors will assist the control of systemvoltages.

The daily loading cycle will also affect the placement of this reactivecompensation. Even during full load, many lines will require permanentlyconnected EHV reactors. At light loads, additional reactive compensationmay be needed to limit terminal voltages rises.

One prior art reactor design utilizes a shell yoke. The basic elementsof such a prior art reactor are shown in FIG. 8. A coil 86 is made inthe form of a hollow cylinder surrounded by a laminated shell yoke 87.This coil may be provided with a high voltage lead 84 at one end thereofand with a lowvoltage lead 85 at the other end thereof. Since gooddesign dictates that the yoke 87 be at ground potential it is necessarythat the high voltage end of the coil be adequately insulated from theyoke 87.

Simultaneously, this spacing must be kept small if the flux lines are tobe maintained in the preferred orientation; that is, parallel to thecoil axis. Such requirements generally impede the use of this design forextra high voltages. Furthermore, in designing reactors, it is foundthat the rated volt-amperes (El) are proportional to the product 3 Vwhere:

B is the magnetic flux density in the coil V is the volume of themagnetic field.

It is obvious that for a compact design it is preferable to use largevalues of magnetic flux density, ,8. The air core, shell type reactor ofFIG. 8 precludes the use of high values of the magnetic flux densitybecause of eddy current losses in the coil windings. Consequently, thevolume, V, is large and the flux density, B, is relatively low. Also,the coil is physically large and many of its turns are exposed to nearlythe full value of magnetic field. These factors result in a reactorwhich can have high resistive and eddy-current losses in the coil.

The gapped-core shell type variation is illustrated in FIG. 9 andconsists of the addition of an interrupted grounded iron core 88inserted in the center of the coil 86. This core 88 is magneticallycoupled to the yoke 87. Pieces of stiff non-magnetic filler material 89are inserted in the core interruptions to maintain the core segments 79in spaced relationship. This design permits the use of values of Bapproaching those that will cause the iron to saturate. Hence, thevolume, V, is smaller. This design does not solve the high voltageinsulation problem between the coil and yoke nor does it achieve gooddistribution of transient overvoltages.

The present invention avoids entirely the insulation problem between theelectric and the magnetic circuits. The device operates at high valuesof B which permits a further reduction in the weight and physical sizeof the reactor. The consequent reduction in size not only reduces thecost but also reduces electrical losses and the inherentmagnetostrictive noise. The novel design employed in utilizing thepresent invention further permits the application of powerfulcompressive forces in the direction of the induced magnetic field toinsure the mechanical stability of the assembly and to reduce acousticalnoise.

Additionally the invention reduces the magnetic leakage flux whileproviding improved and uniform voltage distribution of surges orimpulses, thereby eliminating local areas of high voltage stress.

Broadly speaking these and other advantages and features are achieved ina reactor by providing a pair of parallel windings around a magneticcircuit comprising a pair of magnetic returns coupled by insulatingmagnetic core legs, and electrically and progressively coupling thewindings to the insulated core legs to provide systematic and controlleddistribution of the impressed voltage along the legs.

A reactor built in accordance with the present invention is shown as acut away view in FIG. 10. This reactor comprises a housing 90, mountedon skids 94 on a pad 95. Passing through the top of tank 90 to theinterior thereof is a high voltage bushing 91 and a low voltage bushing92. These bushings may be of a conventional condenser type and arematched mechanically, thermally and electrically to the operatingelement 97 contained within the housing 90 affixed to the sides of tank90 and having passageways connecting with the interior thereof are aplurality of hollow core radiators 93. A suitable insulating fluid 96 isprovided within the tank 90 in sufficient amount to cover element 97 andto circulate by convection through radiators 93. These convectioncurrents are established in the fluid by heating of element 97 whenpower is applied thereto; they may be further assisted by forced fluidpumping. In addition the tank is provided with the usual associatedequipment (not shown) normally found on reactors. This equipmentincludes thermometers, alarm circuits, pressure relief devices, entranceand inspection ports, drain valves and the like.

One novel aspect of any reactor built in accordance with the presentinvention resides primarily in the operating element 97 which is shownin greater detail I in FIGS. 11,12,113 14 and 15.

This element 97 basically comprises a magnetic circuit as shown in FIGS.l1, l2, l3, 14 which is composed of a pair of laminated magnetic returns100 and 101 coupled together by a pair of insulated core legs 102 and103. Each insulated core leg comprises a plurality of core segments 104electrically isolated from one another by disks 106 and spacers 107.Each core segment is built up of strips as discussed in conjunction withFIGS. 3 and 4. Each core segment 104 used in this reactor is preferablyshaped so that the mutually opposed surfaces of adjacent core elementsare spaced apart by a distance which is uniform over said surfacesexcept in the peripheral regions thereof and which, in said peripheralregions, increases towards the exterior. Such shaping insures uniformelectric and magnetic field patterns over most of the gap volume whileeliminating undesirable field concentrations, electric as well asmagnetic, in the peripheral regions.

Each core 104 is surrounded by a current-carrying coil 108. These coilsare electrically connected to each other and to the core segment whichthey surround. Each coil-core pair is electrically isolated from oneanother by disks 106 and spacers 107. These disks and spacers furtheraid in positioning the cores and coils in spatial relationship. Thesedisks and spacers may be made of any suitable insulating material suchas impregnated paper products, pressboard or inorganic dielectrics. Eachdisc 106 may, in turn, be surrounded by an equipotential hoop 110 whichwhen properly electrically connected to the coils and cores on eitherside thereof will assist in the distribution of surge or impulse voltageacross the element 97.

The entire assembly is held together, in compression, by a plurality oftension members such as tie bolts 98 which pass through suitablebrackets 99 affixed on each magnetic return 100, 101. In order toequalize the compressive forces applied to the assembly by these bolts98 a dummy coil 111 made up of suitable insulating material may beprovided around the central core 104A. In most embodiments this centralcore 104A and the dummy coil 111 can be advantageously eliminated.Additionally equalizing may be provided by making the spacers 107 of aresilient material capable of permitting some lateral movement betweeneach coil and its respective core.

Alternatively, the core assemblies and coil assemblies can be mademechanically independant, though clamped between the same magneticreturns, by providing independent dielectric spacers for core and coils.

When the assembly 97 is placed in the housing 90 such that the magneticcircuit is parallel to the base of the tank the spacers 107 areperpendicular thereto and a free vertical path 118 is provided betweenthese spacers 107. This path is such as to permit free flow of theinsulating fluid 96. Passage of the fluid along these paths 118 coolsand insulates the coils and cores. Because of unavoidable losses in theassembly heating of the assembly occurs. This heat is transferred to thefluid 96 by conduction. When the fluid in path 118 becomes sufficientlywarm convection currents will be set up in fluid such that it risesalong paths 118 to the top of radiators 93, downward through theradiators as it cools and into the tank again at the bottom. Thisestablishment of these convection currents cool the assembly and keep ita predeterminable temperature.

FIG. 15.

As noted previously each coil 108 is electrically connected not only toeach adjacent coil but also to its respective core 103. Since theseconnections can be series, series parallel or parallel a briefdiscussion should not be given in conjunction with FIGS. 16, 17, 18, 19and 20. FIGS. 16 and 17 show the coils 108 on each deck electricallycoupled to adjacent coils so as to provide two parallel windings on eachinsulating core leg to result in a total of four parallel windings. Asshown the high voltage is introduced to the central point of thecombination via lead 109. By introducing the high voltage into thecenter of the assembly, interconnecting each coil to its adjacent coreas shown in FIG. 17 while insulating each core segment from the nextsegment in the stack, the frame yoke may be eliminated since themagnetic flux is confined to the magnetic returns 100 and 101 and theinsulating core legs 102 and 103. By eliminating the yoke and byconnecting the coils to the cords the voltage insulation problemexisting in the prior art between the winding and the yoke is alsoeliminated. The low voltage power is extracted via lead 105. We havefound that a high voltage reactor built in accordance with the presentinvention can be lighter in weight and considerably smaller in overallsize than a conventional reactor of the same voltage and power rating.Even more important, its insulation reliability is inherently higher.

FIGS. 18 and 19 show the coils 105 serially connected between legs. Thiswiring arrangement is not preferred however because it produces only twoparallel windings for the entire unit and under transient impulsevoltage conditions has less desirable characteristics.

It should of course be understood that in either event the winding senseof coils on each disk must be such as to cause the magnetic field totravel in a closed loop as indicated by arrows 114.

In either case the difficulties associated with prior art devices whenused for high voltage duty are avoided and a progressive, systematic,and preferably uniform distribution of voltage is achieved across eachinsulating core leg from the mid-point thereof to each magnetic return.This systematic voltage distribution is also achieved under surgeconditions due to the excellent voltage division accorded by the largeinter-core capacities.

Excellent electric field distribution is obtained, in accordance withthe invention, by ensuring that such core segment 104 acts as anequipotential plane" in which the electric charge introduced by surgesor otherwise may be distributed rapidly over the plane while suppressingundesirable eddy currents. A suitable construction is shown in FIG. 26,wherein the metal laminations 50 are electrically connected to eachother along one edge only, while remaining electrically insulated fromone another at all other points, by means of a suitable weld 300 alongone side of the core segment 104.

FIG. 20 shows a modification of the coils 108, their interconnection andthe deck. Here the coil 108 surrounding each core segment 104 is dividedinto two halves. One half 119 is wound in one direction. The core 104 isthen connected to the center point between the two coil halves. It is,of course, necessary that each coil half be insulated from the otherhalf. Additionally the deck 106 can be a laminated structure built up oftwo sheets of suitable insulating material 130, and 131 having aconducting grid 116 sandwiched therebetween. This grid 116 is configuredto minimize eddy currents. Such a sandwiched disk could be used witheach of the configurations described above including the transformer.Insertion of this conduction grid 116 acts to capactively couple eachcore segment to its adjacent core segments to further improve thevoltage surge and impulse response of the unit. This view illustratesstill a further modification that could be utilized in any of theabove-described devices. In this modification the equipotential hoop isreplaced with an encompassing hemispherical ring formed of a insulatingmaterial having a conducting coating deposited thereon.

Returning momentarily to FIG. 15, additional features of the baffleswill be discussed. These baffles 125 are shown shaped to approximatelyconform to the electric equipotential field lines existing betweenequipotential hoops of like voltage on each insulating core leg. Toassure that any generated conducting hydrocarbon chains are broken thebaffles are filled with a suitable number of randomly placed foils 126.To provide adequate flow, a multiplicity of orifices 127 are provided ineach bafile. This combination of orifice, foils and baffles creates agreat deal of turbulence in the fluid and this prevents the formation ofdeleterious conducting chains.

It should be noted that a single large opening 124 is provided for thehigh voltage lead 109.

FIG. 15 also illustrates a modification that may be found desirable.This modification comprises the addition of compression springs 128 onthe end of each tie rod 98 to assure that a constant tension is appliedacross the insulating core legs at all times. A further modification,now shown, is the additional cylindrical insulation over the tensionrods 98.

It should now be obvious to those skilled in the art that the inventionnot only provides an improved reactor capable of EHV duty but does sowith significant savings in weight and cost. The present invention thuspermits the design of a reactor of significantly smaller size since thefull operating voltage is applied so that each portion of the windingbears only its proportional part of the total voltage under both normaland transient conditions. Furthermore by connecting each segment of theinsulated core to the windings the need for extensive insulation betweenhe coils and the core segments is eliminated. This coupling of coresegment with surrounding coils uniformly varies the voltage down eachleg such that the potential gradient is constant down each leg thusmaking maximum use of the leg length for insulating purposes.

Since core segment, insulating spacers and coils are identical the unitlends itself to mass production methods and economy of manufacture.

Since the insulating material provides the function of electricalinsulation as well as mechanical support for both cores and coils thereis a large value of inter-core and inter-coil capacitance whichcontributes to the uniformity of the voltage distribution along each legunder impulse conditions. This diminishes the impact of high voltagetransient stress in the unit.

Utilization of an insulating core provides additional advantages in thatit provides accurate control of the reactor inductance. Additionally,this concept, by permitting uniform application of the magnetizingampere turns over the whole of the insulating cores, reduces magneticleakage flux.

The described mechanical configuration provides advantages in that theunit, when mounted horizontally, can be cooled by natural conventioncurrents while simultaneously applying large compressive forces to theunit thereby reducing acoustical noise.

It should be obvious that other modifications and adaptions can now bemade to the described reactor. For example, the reactor could be adaptedto three phase operation by sjing three legs each of which has a highvoltage input lead in the center.

Still further as shown in FIG. 21, the shaped core 104 can be encased ina molded solid dielectric 117 which is shaped in the form of a spool.The coils are then wound on the spool and the spools stacked to form theinsulating core legs of the unit. If desired the conductive grid 116 ofFIG. could be inserted between each spool and connected to anequipotential hoop surrounding the spool interface.

Referring now to FIGS. 22, 23a, a preferred embodiment of an insulatedcore reactor built in accordance with the principles of the presentinvention is shown and designated generally by the numeral 180. Thisreactor comprises a magnetic circuit which is developed through endyokes 182 and 184, and magnetic columns or legs 183 and 185 each ofwhich comprise a plurality of core segments such as 186. Each coresegment 186 is surrounded by coils 189. Separation of the core segmentsis achieved through insulating layer 191. The entire assembly isenclosed in a tank 193 which holds the end yokes 182 and 184 and thelegs 183 and 185. Connection is made to the reactor by a high voltagebushing 195 containing a high voltage line 220. For ease of movement,the reactor 180 is mounted on a skid 197. In FIG. 23a the high voltageterminal 301 and adjacent core segments 188, 190 and 192 are shown indetail. Each core segment such as 190 is surrounded by a set of fourcoils 194, 196, 198 and 200 electrically connected such that the coils194 and 196, and 198 and 200 each form a parallel set by connections202, 204, 206 and 208. The resulting parallel coil combinations 194, 196and 198, 200 are then connected in series by connector 210 which alsoprovides a connection to the core segment 190. By this parallelarrangement no potential difference exists between corresponding turnsin the space between coils 194 and 196 and the space between coils 198and 200. Ac.- cordingly, oil or any other suitable cooling medium can bereadily circulated throughout these spaces between parallel coil setswith little risk of insulation failure in these areas.

Unlike the previously described induction apparatus, no dummy centercore is used. Moreover, as shown in FIG. 22, the core segments 186 areof an even number and thus the high voltage terminal 301 is located in aplane between the two center core sections 188 and 190. Thus aspreviously described, the voltage would then decrease from the highvoltage connection in both directions to the grounded end yokes 182 and184. It should be noted that the end yokes 182 and 184 are extended overthe soils in order to carry the total magnetic flux existing in the coilwinding area. This feature has the desirable effect of keeping themagnetic field from curving at the end coils and thus reducessignificantly any eddy-current loss which would otherwise be present. Inaddition, the end yokes themselves can not be used for clamping both thecoil and core assemblies thereby minimizing vibration and eliminatingextra clamping devices.

For purposes of explanation, it may be assumed that the voltage dropacross each set of four coils is equal to V= V V where V, the potentialat connector 202 V the potential at connector 206 and thus the voltagedrop across any parallel set would be equal to FIG. 23b shows in graphform the voltage level decrease corresponding to the coil sets 194, 196and 198, 200. The voltage decrease through the parallel coil set 194,196 decreases from a maximum value of V at the outer extremity to avalue of at the inner extremity which is likewise the potential at whichthe core segment is maintained. From the inner extremity of the parallelcoil set 198, 200 to the outer extremity the voltage continues todecrease from a value of to V As shown in FIG. 23a, the outer extremityof coil set 198, 200 is then connected to the outer extremity of thenext coil set 216, 218 through connector 209 and the same voltagedecrease then takes place throughout the remaining coil sections andcore segments until eventually the respective grounded yoke 184 isreached. Similarly the high voltage terminal 301 is connected to thecoils surrounding core segment 188 and the voltage distributiondecreases similarly to its respective grounded end yoke 182.

Since the voltage decrease throughout each parallel set of coils isequal to the difference in potential between the inner extremity ofparallel coils set 198, 200 and parallel coil set 216,

18 V2 V1(Or Again, where the word coil is used in this description ofthe invention, each coil can be composed of a plurality of smallercoils, e.g., pancake coils, with or without their conductors transposedin some predetermined sequence to further reduce the losses caused byeddy currents flowing in the conductors.

Referring now to FIGS. 24, 25a, 25b, 25c and 25d, the high voltageterminal 301 may be fabricated in the following manner. Using the sameconductor as that used in the coils, namely, a thin copper or aluminumband 302 approximately one-half inch wide and having a thin insulatingcoating (not shown), a coil is formed in the usual manner except that itis wound together with a filler strip 303 of insulating material so asto space adjacent turns of the coil from one another. After thedouble-coil has thus been wound, a weld 304 is made radially across thecoil so as to short circuit all turns. At the opposite side of the coila cut 305 is made creating a radial gap. The exposed ends of thewindings 302 are then spread apart and the filler strips 303 cut away toa depth of approximately 3 inches on both sides of the gap.'These arethen replaced by filler strips 306 having a length sufficient to fillthe space to provide a mechanical but insulating joint across theelectrical gap. The resultant structure is given rigidity by overwindingthe entire coil with insulating tape 307.

The high voltage terminal is electrically connected by connector 209 tothe outer extremities of the parallel coil set on either side of theinsulation 19lb.

As a consequence of the thin planar structure of the high voltageterminal, its capacitance to the grounded tank is small in comparison toits capacitance to the coil and core capacitance chains.

Under surge or transient conditions when a sudden voltage increaseappears on the high voltage line 220, the increase in potential isquickly spread throughout the high voltage terminal 301 and thence downthe series capacitance chains to the grounded yoke presented by both thecoil assemblies and the core assemblies.

The coil pattern which is used in the high voltage terminal 301 need notbe restricted to the particular construction described and a number ofother construction of this general geometry could be employed to producethe same result. As previously stated, the function of the high voltageterminal is to provide a low surge impedance equipotential plane whichis connected to the high voltage input and has low capacitance to groundand which has negligible eddy-current losses and which is intimatelycoupled, capacitively and conductively, to the core and coil assemblies.

Having thus described the principles of the invention, together withseveral illustrative embodiments thereof, it is to be understood that,although specific terms are employed, they are used in a generic anddescriptive sense, the scope of the invention being set forth in thefollowing claims.

We claim:

1. Electromagnetic induction apparatus for highvoltage, high-poweroperation at a voltage V comprising a tank containing insulating fluidand, mounted therein, the following elements:

a. at least two magnetic columns each having a first end plane, a secondend plane,'a midplane and a high-voltage terminal in said midplane b. afirst magnetic yoke magnetically connecting said first end planes 0. asecond magnetic yoke magnetically connecting said second end planes eachof said magnetic columns comprising a series of similar core elementsseparated by a series of n similar electrically insulating layers eachof said core elements being composed of a laminate of ferromagneticstrips separated by insulation of a thickness sufficient only to preventeddy currents,

each of said similar electrically insulating layers having a thicknesscapable of supporting a voltage of at least 2V/n,

d. at least one winding surrounding each of said magnetic columns andhaving a midpoint which is connected to said high-voltage terminal, saidwinding being electrically connected to each of said core elements atrespective neighboring portions thereof so that the electric potentialof each core element is maintained close to the potential of thatportion of the winding nearest to the core element in question, wherebysaid winding may be and therefore is wound in proximity to said magneticcolumn and e. high voltage bushing means providing a conductive pathbetween each high-voltage terminal and the region external to said tank.

2. Apparatus according to claim 1, wherein the electric potential ofeach part of said winding is always in the vicinity of the electricpotential of that core element nearest the part in question, wherebysaid winding may be and therefore is wound closely about said magneticcolumn.

3. Apparatus according to claim 1, wherein the mutually opposed surfacesof adjacent core elements are spaced apart by a distance which isuniform over said surfaces except in the peripheral regions thereof, andwhich, in said peripheral regions, increases towards the exterior.

4. Apparatus according to claim 1, wherein adjacent metal strips in eachof said laminated core elements are connected by conductive meansoccupying a negligible portion of the inter-strip gap.

5. Apparatus according to claim 1, wherein said winding constitutes twoparallel power circuits for each magnetic column, and said high voltageterminal consists of a conducting member in the midplane of eachmagnetic column and conductively connected to the midpoint of saidwinding for the purpose of distributing more uniformly both surge anda-c voltages over the winding and magnetic columns while at the sametime suppressing eddy currents in said distributing means.

6. Apparatus according to claim 1, wherein said high voltage terminal isa thin planar conductive, but eddycurrent-suppressing element andtherefore has low shunt capacitance to ground and high capacitance tothe winding on either side so that the distribution of surge voltageslengthwise along said magnetic columns and their surrounding windings isrendered more uniform because of the cominance of series capacitancesalong the column with respect to the shunt capacitances to ground.

7. Apparatus according to claim 1, wherein at least one conductive leadis connected to the winding at a point removed from the high voltage endfor the purpose of measuring the potential of the high voltage terminalor for transferring electric power at a correspondingly lesser voltageor both.

8. Apparatus according to claim 1, wherein said high voltage terminalcomprises a thin planar conductive but eddy-current-suppressing elementsupported in the midplane of each magnetic colunm and wherein said highvoltage terminal is connected to the junction of said high voltagebushing and said midpoint of said winding and therefore possesses a highseries capacitance to the portion of the winding and magnetic posed ofdielectric material of Youngs modulus in excess of 10 pounds per squareinch but are covered on each surface subjected to compressive forces bya negligible thickness of dielectric material of relatively lower Youngsmodulus.

12. Apparatus according to claim 4 wherein said laminated core elementsconstitutes a low impedance equi-potential plane with negligableeddy-current loss under high a-c magnetic flux densities whichdistributes both operating and transient voltages uniformly over thesurfaces of abutting electrically insulating layers.

1. Electromagnetic induction apparatus for high-voltage, highpoweroperation at a voltage V comprising a tank containing insulating fluidand, mounted therein, the following elements: a. at least two magneticcolumns each having a first end plane, a second end plane, a midplaneand a high-voltage terminal in said midplane b. a first magnetic yokemagnetically connecting said first end planes c. a second magnetic yokemagnetically connecting said second end planes each of said magneticcolumns comprising a series of similar core elements separated by aseries of n similar electrically insulating layers each of said coreelements being composed of a laminate of ferromagnetic strips separatedby insulation of a thickness sufficient only to prevent eddy currents,each of said similar electrically insulating layers having a thicknesscapable of supporting a voltage of at least 2V/n, d. at least onewinding surrounding each of said magnetic columns and having a midpointwhich is connected to said highvoltage terminal, said winding beingelectrically connected to each of said core elements at respectiveneighboring portions thereof so that the electric potential of each coreelement is maintained close to the potential of that portion of thewinding nearest to the core element in question, whereby said windingmay be and therefore is wound in proximity to said magnetic column ande. high voltage bushing means providing a conductive path between eachhigh-voltage terminal and the region external to said tank.
 2. Apparatusaccording to claim 1, wherein the electric potential of each part ofsaid winding is always in the vicinity of the electric potential of thatcore element nearest the part in question, whereby said winding may beand therefore is wound closely about said magnetic column.
 3. Apparatusaccording to claim 1, wherein the mutually opposed surfaces of adjacentcore elements are spaced apart by a distance which is uniform over saidsurfaces except in the peripheral regions thereof, and which, in saidperipheral regions, increases towards the exterior.
 4. Apparatusaccording to claim 1, wherein adjacent metal strips in each of saidlaminated core elements are connected by conductive means occupying anegligible portion of the inter-strip gap.
 5. Apparatus according toclaim 1, wherein said winding constitutes two parallel power circuitsfor each magnetic column, and said high voltage terminal consists of aconducting member in the midplane of each magnetic column andconDuctively connected to the midpoint of said winding for the purposeof distributing more uniformly both surge and a-c voltages over thewinding and magnetic columns while at the same time suppressing eddycurrents in said distributing means.
 6. Apparatus according to claim 1,wherein said high voltage terminal is a thin planar conductive, buteddy-current-suppressing element and therefore has low shunt capacitanceto ground and high capacitance to the winding on either side so that thedistribution of surge voltages lengthwise along said magnetic columnsand their surrounding windings is rendered more uniform because of thecominance of series capacitances along the column with respect to theshunt capacitances to ground.
 7. Apparatus according to claim 1, whereinat least one conductive lead is connected to the winding at a pointremoved from the high voltage end for the purpose of measuring thepotential of the high voltage terminal or for transferring electricpower at a correspondingly lesser voltage or both.
 8. Apparatusaccording to claim 1, wherein said high voltage terminal comprises athin planar conductive but eddy-current-suppressing element supported inthe midplane of each magnetic column and wherein said high voltageterminal is connected to the junction of said high voltage bushing andsaid midpoint of said winding and therefore possesses a high seriescapacitance to the portion of the winding and magnetic column on eitherside with low shunt capacitance to ground.
 9. Apparatus according toclaim 1 wherein said magnetic columns and said surrounding windings aresubjected to compression forces from said first and second magneticyokes.
 10. Apparatus according to claim 9 wherein said magnetic columnsand said surrounding windings are subjected to compression forces fromsaid first and second magnetic yokes but are otherwise mechanicallyindependent of each other.
 11. Apparatus according to claim 10 whereinthe electrical insulating layers are predominately composed ofdielectric material of Young''s modulus in excess of 106 pounds persquare inch but are covered on each surface subjected to compressiveforces by a negligible thickness of dielectric material of relativelylower Young''s modulus.
 12. Apparatus according to claim 4 wherein saidlaminated core elements constitutes a low impedance equi-potential planewith negligable eddy-current loss under high a-c magnetic flux densitieswhich distributes both operating and transient voltages uniformly overthe surfaces of abutting electrically insulating layers.